feature/leetcode-5-31

src/algorithms/trap_rain_water/app.py

@@ -0,0 +1,47 @@
+from typing import List
+
+
+class TrapRainWater:
+    """
+    Source : https://leetcode.com/problems/trapping-rain-water/
+
+    Given n non-negative integers representing an elevation map where the width of each bar is 1,
+    compute how much water it is able to trap after raining.
+
+    Example:
+
+    3                               +
+    2               +   #   #   #   +   +   #   +
+    1       +   #   +   +   #   +   +   +   +   +   +
+
+    + = bar
+    # = water
+
+
+    Input: [0,1,0,2,1,0,1,3,2,1,2,1]
+    Output: 6
+    """
+
+    def function(self, height: List[int]) -> int:
+        '''
+        O(n)
+        Remember the rule: No matter how large an array is,
+        max_water_height == min(left_bar, right_bar)
+        '''
+
+        l = tot = l_max = r_max = 0
+        r = len(height) - 1
+
+        while l < r:
+            if height[l] <= height[r]:  # Leftside is shorter
+                if height[l] > l_max:
+                    l_max = height[l]
+                tot += l_max - height[l]
+                l += 1
+            else:  # Rightside is shorter
+                if height[r] > r_max:
+                    r_max = height[r]
+                tot += r_max - height[r]
+                r -= 1
+
+        return tot

src/algorithms/trap_rain_water/test_app.py

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+import unittest
+from .app import TrapRainWater
+
+
+class TrapRainWaterTest(unittest.TestCase):
+    def setUp(self):
+        self.tester = TrapRainWater()
+
+    def test_case_1(self):
+        input_ = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]
+        exp = 6
+        self.assertEqual(self.tester.function(input_), exp)
+
+    def test_case_2(self):
+        input_ = [6, 2, 5, 2, 1]
+        exp = 3
+        self.assertEqual(self.tester.function(input_), exp)